Methods of measuring a ligand qualitatively and quantitatively by causing or inhibiting agglutination of particles using insoluble carrier particles are performed easily at high sensitivity. Therefore, various measurement reagents have been developed using ligand-receptor reaction principles such as immunoreactions between antigens and antibodies or binding reactions between complementary nucleic acid chains.
The agglutination method is a method of measuring the degree of agglutination of receptor-sensitized carrier particles due to a crosslinking reaction via ligands to be measured, and the method has been in practical use for many measurement items because of the background of the need for sensitive measurement of trace substances.
On the other hand, the agglutination inhibition method is a method of measuring the ratio of agglutination inhibition, by a ligand to be measured, between ligand- or ligand-like substance-sensitized carrier particles and a receptor. In the case of the agglutination method, multiple kinds of specific receptors are required except for unusual kinds of ligands, while in the case of the agglutination inhibition method, a measurement system can be constructed using only one kind of a specific receptor.
Moreover, as for the immunoreactions, the agglutination inhibition method is advantageous, for it does not cause hook effect (superficial reduction of reaction, a phenomenon caused by excessive antigen). However, the method has been employed, in most of the cases so far, for a low-molecular-weight ligand, such as hapten, which multiple kinds of receptors cannot be used for.
Application of the agglutination inhibition method has been limited because controlling the range of measurement concentration is difficult compared with the agglutination method. In the agglutination method, agglutination progresses in proportion to the amount of the ligand, and hence, the range of measurement can be easily expanded only by enhancing the increase of sensitivity at the high-concentration range as shown in, for example, Patent Document 1 which is related to a latex-enhanced immuno-agglutination method.
On the other hand, as for the range of measurement of the agglutination inhibition method where the agglutination inhibition reaction progresses in proportion to the amount of ligand, the upper limit of measurement at a high-concentration range of the ligand basically depends on the degree of the initial agglutination (agglutination in the absence of the ligand), while the lower limit of measurement at a low-concentration range of the ligand depends on the sensitivity of the agglutination inhibition reaction. Therefore, it is necessary to consider both factors together.
Patent Document 2 discloses a method using a non-specific agglutination-promoting substance in measurement of hapten, as a method for enhancing the initial agglutination in the agglutination inhibition method. This method can measure a ligand at a high-concentration range and extend the upper limit of measurement, but the sensibility to the agglutination inhibition reaction is lowered because of enhancement of agglutination which is irrelevant to a specific reaction between the ligand and receptor, resulting in significant deterioration of detectability at a low-concentration range.
Meanwhile, Patent Document 3 discloses an agglutination inhibition method using a carrier sensitized with a single monoclonal antibody and a carrier sensitized with an antigen. In this method, the detectability at the low-concentration range is improved without impairing the sensibility to the agglutination inhibition reaction, by using a carrier particle which carries a ligand and a carrier particle which carries a receptor, respectively, and by optically enhancing the degree of sensitivity changes. However, in this method, the reaction itself between the antigen and the antibody is not enhanced, and hence the effect of expanding the range of measurement cannot be achieved. Also in the agglutination inhibition method proposed in Patent Document 4, where antigen-sensitizing latex, an antibody, and a receptor specific to the antibody are used, the agglutination is enhanced only by specific reactions between the antigen and the antibody and between the antibody and the receptor, and hence, expansion of the range of measurement, relative to the conventional methods, can be achieved. Protein A used in the example as a receptor, however, has reactivity to any antibodies (IgG), and hence, this method is not suitable for measurement of serum or plasma samples containing the IgG In addition, the anti-mouse IgG rat monoclonal antibody is not a commonly used material and may be difficult to obtain. Moreover, the measurement principle itself is through the biphasic reaction of the antigen-antibody and the antigen-receptor, and the performance may be lowered depending on the measurement device and the method.
As described above, neither an agglutination-inhibition assay nor a reagent for agglutination-inhibition assay, which have opposing properties of strong initial agglutination and high sensitivity to the agglutination inhibition reaction, has been established, and it has been difficult to measure, with high reproducibility, normal ligands, for example, ligands having multiple receptor binding sites, at high sensitivity in a wide range by conventional assays.
Patent Document 1: JP 2004-191332 A
Patent Document 2: JP 05-000665 B
Patent Document 3: JP 59-173760 A
Patent Document 4: JP 2001-337092 A